Interventional radiology uses medical imaging systems such as computed tomography, magnetic resonance (MR), X-ray, and fluoroscopy for therapeutic as well as diagnostic purposes. Healthcare providers use mostly fluoroscopy to visualize a catheter placed within the vasculature of a patient to assist in guiding the catheter to a remote site within the body. Catheters so positioned can be used for injecting a contrast agent to image the vasculature of specific organs, analyzing local chemistries, retrieving samples (e.g., a tissue biopsy), effecting therapeutic procedures such as cryosurgery, delivering a therapeutic agent, blocking of an artery feeding a tumor, or the like. The goal of the procedure is to deliver the working end of a catheter to a specific internal site within the body of the patient.
The vascular system is used to contain the catheter and to act as the conduit along which the catheter progresses. Access to endovascular space is achieved via a puncture with a needle through the skin and through the wall of the vessel, either at the groin or, less frequently, from the axillary area. Typically, a catheter tip is threaded to the destination vessel by manually steering it from the point of entrance, taking advantage of the torque of the relatively stiff catheter with a curve at the tip to enter side branches. The tip of the catheter is advanced by pushing and rotating the catheter at the point of entrance through the skin. The progress of the tip of the catheter within the vascular system is observed on a fluoroscopic monitor when small amounts of radio-opaque contrast material are injected into the bloodstream through the catheter. In addition, guidewires are often used to help pass the catheter, especially in tortuous or sharply angled vasculature.
Although skilled angiographers can reach most destination vessels, the time and effort required to guide the tip of the catheter to the destination vessel, the radiation dose to the patient and staff due to the fluoroscopic imaging required to monitor the progress of the tip of the catheter through the vasculature, the burden of contrast materials introduced into the patient and possible reactions thereto, as well as other complications inherent in the procedure of manually guiding the tip of a catheter, make these procedures difficult and risky. In addition, some vessels cannot be successfully navigated due to the patient's anatomy. Steep angulation of branches or marked tortuosity of vessels can prevent access, especially at the hands of the less-skilled physician.
The position of a catheter in a vessel can be monitored using magnetic resonance (MR) imaging techniques. Briefly, MR imaging is a technique in which an object placed in a spatially varying magnetic field is subjected to a pulse of radio frequency radiation, and the resulting nuclear magnetic resonance signals are combined to give intensity-modulated cross-sectional images of the object. MR imaging systems generally include a large magnet for generating a magnetic field. A patient or object being analyzed is exposed to the magnetic field of the magnet. Hydrogen nuclei (protons) or other biologically significant, nonzero spin nuclei, e.g., .sup.31 P, .sup.23 Na, .sup.13 C, in the magnetic field resonate when exposed to radio waves of a correct frequency. For imaging purposes, the strong uniform magnetic field of the magnet is selectively altered in one or more directions, preferably by small magnetic fields produced by three separate gradient coils associated with the magnet. Current passing through the gradient coils linearly alters the z component of the magnetic field of the magnet in directions controlled by the gradient coils. Signal transmission and reception are produced through use of a radio frequency (RF) transmitter coupled to a transmitting coil or antenna within the imaging unit and an RF receiver coupled to a "receiver coil" also located in the imaging unit. The receiving coil is positioned as close to the patient or object as possible for maximum imaging sensitivity. The patient or object is often surrounded by a body coil that serves both as transmitting and receiving antennae. Alternatively, the body coil can be used as a transmitting antenna only, and a separate surface coil is used as a receiving antenna. The surface coil can usually be placed closer to the tissues or object under examination than a single body coil. An RF oscillator generates radio waves of different frequencies. By controlling the magnetic field in a known way through a switching system that controls the current in the gradient coils, and by generating radio waves of a select frequency, the exact location at which the patient's body or the object is imaged can be controlled. When the frequency of the RF signal is set for the exact value of the magnetic field, resonance occurs. Precession of the excited nuclear magnetic moment leads to induction of small currents in the receiving coil. The induced currents are detected to produce an output signal dependent upon the number of protons involved in the resonance and tissue-specific parameters. The output signal from the RF receiver is processed by a computer system to produce an image display so that the position of the coil can be determined. See, e.g., Brown et al. (1995) MRI: Basic Principles and Applications (Wiley-Liss, New York).
Thus, for example, a method for measuring the position of a small RF coil with respect to the coordinate system of an NMR imager has been described in U.S. Pat. No. 4,572,198. Passive tracking of a catheter with near-real-time two-dimensional angiography has also been described. See, e.g., Bakker et al. (1997) Radiology 202:273-276; Bakker et al. (1996) Mag. Reson. Med. 36:816-820, Kandarpa et al. (1993) J. Vac. Interv. Radiol. 4:419-427; Leung et. al. (1995) Amer. J. Radiol. 164:1265-1270; Dumoulin, C. L., et al. (1993) MRM 29:411-415). In addition, intravascular coils, designed for the vascular imaging of atherosclerotic plaques using a 1.5 T scanner, have been described in Martin et al. (1994) Magn. Res. Med. 32:244-249.
However, the ability to use MR imaging techniques to monitor the position of a catheter or other device within the endovascular space of a patient or, for that matter, within a path to any remote destination, does not resolve the difficulty of manually guiding the catheter or device to a destination site that may require, e.g., traversing a tortuous or sharply angled vasculature or other conduit, or guiding the tip of the catheter into small diameter vessels or other branch lines.
Accordingly, there remains a need in the art for a method by which the travel of a catheter or other device to a remote location within endovascular space or along a path to the remote location, can be guided by remote control. In addition, there is a need in the art for an apparatus to effect such a method and to a catheter useful with such a method.